Continuous satellite two-way time transfer using commercial modems

Satellite two-way time transfer has traditionally been used as a periodic measurement of the offset between two clocks. It is often used as a secondary measurement to GPS or a sole means of time recovery with performance that is superior to GPS time transfer. In this paper, we propose a continuous two-way system based on commercial modems where the two-way time transfer is performed in the background of normal data transfer. The intent of such a system is to provide a cost effective means to achieve high accuracy two-way time transfer in the background of an active data channel. The timing functionality is buried in the administrative portion of the communications channel and thus is provided as a byproduct of data transfer. This provides lower cost, lower bandwidth options for users who require sub-nanosecond time transfer between sites. It also provides a continuous record of clock differences with no impact on data transmission for users who have access to a data communications link. This paper begins with a review of the concept of two-way time transfer and time based communications and presents the requirements for implementing two-way systems over a communications channel. The requirements are then satisfied with commercial modems using two examples: an internal implementation and an external implementation. Data is presented for each case and tradeoffs are discussed.